Method for determining the corrosion resistance of anodized aluminum parts



Dec. 20, 1966 J. STONE 3,293,155

METHOD FOR DETERMINING THE CORROSION RESISTANCE OF ANODIZED ALUMINUMPARTS Filed July 12; 1965 2 Sheets-Sheet 1 7'/MER INTEGRA 7' I0 U/V/ TRECORWER F/G. Z

JACK S TO/VE INVENTOR.

4 7' TOR/VEYS' Dec. 20, 1966 J. STONE 3,293,155

METHOD FOR DETERMINING THE CORROSION RESISTANCE OF ANODIZED ALUMINUMPARTS 2 Sheets-Sheet 2 Filed July 12, 1965 5 mwgk K h? m 0 d UnitedStates Patent 3,293,155 METHOD FOR DETERMINING THE COR- ROSIONRESISTANCE OF AN ODIZED ALUMINUM PARTS Jack Stone, Detroit, Mich.,assignor to Ford Motor Company, Dear-born, Mich, a corporation ofDelaware Filed July 12, 1965, Ser. No. 471,283 19 Claims. (Cl. 204-1)This application is a continuation-in-part of my copending applicationSerial No. 67,110 filed November 3, 1960, now abandoned.

This invention relates to a method for evaluating metal oxide coatingsupon a metal base or substrate. More specifically, this inventionrelates to a method for evaluating the corrosion resistance of suchcoatings electrochemically by contacting the coating opposite the metalbase with an aqueous electrolyte, and evaluating the electricalresistance of such coating with respect to time over a predeterminedperiod of time under impressed voltage between the electrolyte and themetal base.

In a preferred embodiment of this invention, the coating is placedwithin an electrical circuit wherein the metal base serves as a cathode,said circuit comprising said base, an anode, an aqueous electrolyte incontact with said coating and said anode, and a significant electricalresistance in series electrical connection with said coating exterior tosaid electrolyte, impressing an electrical potential upon said circuitcausing a first voltage drop across the exterior resistance and a secondvoltage drop across the coating, and integrating the voltage across saidcoating with time for a predetermined period of time. In anotherembodiment, current through said coating is integrated with time for apredetermined period of time.

While this test method has general application for evaluating thecorrosion resistance of metal oxide coatings upon a metal base, to avoidduplication of description this invention is hereinafter explained indetail with reference to anodized aluminum, a widely used industrialproduct comprising an aluminum base and an aluminum oxide coating uponand formed from such base.

Anodized aluminum is now widely used for automobile trim and in a widevariety of other metal products. The oxide coating may be polished to amirror-like finish but also finds use with other finishes.

In the anodizing process, an aluminum surface is electrochemicallyconverted to a hydrated form of aluminum oxide and conventionally thisconversion is carried out by passing current through an electrolyticcell in which the aluminum object is installed as the anode with asulfuric acid electrolyte and a lead cathode. The relatively porousaluminum oxide coating is conventionally sealed in boiling Water todecrease its porosity and to increase its corrosion resistance.

The corrosion resistance of the oxide coating on an anodized aluminumsurface may vary widely with differences in coating thickness andcoating continuity. The latter may be viewed as including both theuniformity of the original conversion and the quality of the postconversion seal. Minimum thickness may vary considerably from theaverage thickness. Such variance can result from impurities in thealuminum surface subjected to the anodizing process which resist theanodizing process and/or prevent conversion of the aluminum below at arate commensurate with the conversion of other portions of the surface.Except for an almost complete absence of coating in a localized area,these variances in coating depth are not discernable by visualinspection prior to use exposure. Electrochemical examination of oxidecoatings formed by anodizing aluminum, by way of measuring the electriccurrent through the coating after a measured period of electrochemicalattack, has been disclosed by Dr. Roy C. Spooner in an article entitledThe Sealing of Sulfuric Acid Anodic Films on Aluminum, published in theTechnical Proceedings of the 44th Annual Convention of the AmericanElectroplaters Society.

It now has been discovered that one may rapidly and objectively evaluatethe corrosion resistance of an anodized aluminum substrate by installingsuch substrate as the cathode of an electrolyte cell, applying a voltageacross the coating for a predetermined and fixed period of time andintegrating the voltage across said coating through said period of time,i.e. measuring the area beneath the resultant voltage-time curve. v

It is therefore one object of this invention to provide an acceleratedcorrosion test for metal oxide coated metal parts.

A further object is to provide a simple test which can be eflicientlyapplied in controlling the production of anodized aluminum.

A further object is to provide a rapid and quantitative corrosion testwhich correlates closely with field exposure results.

Other objects and advantages of this invention will be made moreapparent as this description proceeds, particularly when considered inconnection with the accompanying drawings in which:

FIGURE 1 is a schematic sectional view of a portion of an electrolyticcell used in determining the corrosion resistance of anodized coatingson aluminum parts;

FIGURE 2 is a schematic diagram illustrating one embodiment ofelectrical circuitry that may be used in carrying out the test method ofthis invention with the electrolytic cell therein shown in section; and

FIGURE 3 is a graphic illustration of recorded voltages across anodizedcoatings in a fixed period of time in which two voltage-time curves arerepresentative of a difference in the corrosion resistance of theanodized coatings on two aluminum sheets.

The oxide coating formed by the anodizing process has a high electricalresistance in comparison With its aluminum substrate which is anexcellent conductor of electricity. By establishing an electrolytic cellon the surface of the anodized coating, one can measure the corrosionresistance of the coating, the corrosion resistance being inverselyproportional to the electrical resistance exhibited with time by thecoating under electrochemical attack. The test is accelerated by usingan aqueous electrolyte solution that becomes slightly corrosive to thecoating in the conduct of the test, e.g. sodium chloride, sodiumchloride containing minor amounts of cupric chloride and acetic acid,acetic acid, etc.

In FIGURE 1 is seen an electrolytic cell 11 which comprises a glass tube12 having a rubber grommet 13 attached to one end. The rubber grommet13, which has an opening approximately one-eighth of an inch indiameter, is placed in firm contact with an anodized surface 14 on analuminum base 15.

An electrode 16, which may be platinum due to its inertness to corrosiveliquids, is placed in the glass tube 12. An electrolyte solution 17 isthen introduced into the glass tube 12 so that the solution 17 willmakecontact with the portion of the surface 14 defined by the opening inthe rubber grommet 13.

An electrolyte solution 17, is prepared by dissolving 190.0 grams ofsodium chloride and 1.0 gram of cupric chloride dihydrate in 4 liters ofdistilled water. The pH of this solution is then adjusted to between 2.8and 3.0 by the addition of acetic acid.

As seen in FIGURE 2, the aluminum base 15 is connected to the negativeterminal of a direct current electric power source 18 to become thecathode of the electrolytic -3 cell. The electrode 16 is connected tothe positive terminal of the power source 18 to become the anode of theelectrolytic cell 11. A resistance 19 and optionally a milliammeter 20are placed in series with the electrode 16 and the power source 18.

The resistance 19 should be of sufiicient magnitude so that asignificant and measurable change in the voltage drop across the coating14 occurs with a corresponding change in current through the coating. Inone preferred embodiment, a 53,000 ohm resistor has been used with aconstant applied voltage from power source 18 of 46 volts. To avoid theuse of unnecessarily sensitive and expensive instrumentation, theresistance 19 should not be below about 100 ohms and preferably notbelow about 10,000 ohms. With most commercial depth anodized aluminumcoatings a potential of at least about volts across the coating isadvisable. This voltage should not be in excess of that at which theelectrochemical deterioration is effected in a reasonably gradual mannerso that the electrical resistance of the coating is dissipated over asignificant time period. In choosing the amplitude of resistance 19,consideration must be taken of the amplitude of the applied voltage andvice versa.

With coatings having a corrosion resistance significantly better thanthe required minimum for a given use, the applied voltage at powersource 18 should be high enough to eifect an electrochemical breakdownof the electrical resistance of that portion of the coating in contactwith the electrolyte within the predetermined test period taking intoaccount the intervening resistance. Electrochemical breakdown of thecoating under test permits relatively unrestricted fiow of electricalenergy through the coating and is evidenced by a marked increase incurrent flow through the coating, observable on milliammeter 2t}, and acorresponding decrease in the electrical resistance of the coating.

The test is initiated by closing a self-releasing, starting switch 21which starts a conventional timing device in timer 22 and via suitableconductors closes contact switches 23 and 24 and causes the appliedvoltage of power source 18 to be impressed upon the circuit. The appliedvoltage results in a first voltage drop across resistance 19 and asecond voltage drop across the coating 14. The resistance of the circuitconductors is insignificant. When the voltage is applied the initialcurrent flow is dependent on the conductivity of the coating 14. As thecurrent begins to flow, hydrogen is discharged at the cathode and theelectrolyte becomes more alkaline, particularly adjacent to the cathode.As the test proceeds, the insulating properties of the coatingdeteriorate and the eifective electrical resistance of the coatingdecreases. The applied voltage is maintained at a constant level andwhen a decrease occurs in the electrical resistance of the coating 14the difference of potential across the coating decreases and aproportionally greater voltage drop occurs across the resistance 19. Thevoltage across the coating 14, and hence the changes therein, isrecorded throughout the test period by a voltage recorder 25, aconventional electronic measuring and tracing device, providing avoltage-time tracing such as shown in FIGURE 3. The use of recorder 25is optional and is here illustrated and described to facilitateunderstanding of the function and purpose for integration unit 26hereinafter discussed.

To obtain an objective evaluation of the corrosion resistance of theoxide coating 14, the voltage drop across the coating is integrated withtime throughout the test period. In practice, an interval of threeminutes has been found short enough to be acceptable in commercialproduction and long enough to give a meaningful test. Obviously, thetime may be extended as an essentially constant current through thecoating results after localized breakdown of the coating is'completedalthough no purpose is achieved by continuing test after a majorproportion of the original resistance has been dissipated. Functionally,the acceptable minimum is a time sufficient to obtain from a coating ofgiven depth an initial measure ment of electrical resistance and ameaningful measurement of its resistance to electrochemical attack.Preferably, this time is at least one minute. This evaluation may bedone manually and/or by routine calculus measurement of the area belowthe voltage-time tracing on the recording chart. As a practical matterthis measurement is made by conventional electronic integra tioninstruments indicated in FIGURE 2 by an integration unit 26. This unitincludes a conventional voltage to frequency converter and counter whichmeasures the voltage-time integral of the input signal over the timeinterval selected. The voltage to frequency converter generates acontinuous pulse train whose rate is proportional to the magnitude ofthe input voltage integrated between each output pulse. One suchconverter that has been satisfactorily employed in conducting such testshad a range of 0 to 100,000 cycles per second. The measurement providedby the counter is a function of volt-seconds integrated over thepreselected time interval and is automatically ecorded. This reading ora selected fraction or multiple thereof provides an objective figure ofmerit for the coating under test. In a plurality of tests the figures ofmerit are proportional to the areas under the corresponding voltage-timetracings in recorder 25 When the selected time interval has expired, aconveri= tional switching mechanism is actuated by timer 22 open= ingswitches 23 and 24 and terminating the test.

By way of example, two anodized aluminum sheets were evaluated with atest unit schematically illustrated in FIGURE 2 using an applied voltageof 46 volts and a resistance at 19 of 53,000 ohms in series with thecoatings being tested. The tracings obtained for each ofthe two testsare superimposed on a single graph and illustrated by lines A and B inFIGURE 3. The corresponding figures of merit obtained from theintegration unit 26 for such tests were 1690 and 860 volt-secondsrespectively. These figures are obtained by dividing the total number ofpulses counted within the predetermined test period by the number ofpulses the given voltage-frequency con verter provides per volt-secondinput. This divisor will depend upon the rating of the converter used.Thus,- if a converter has an output of pulses per volt-second input, thetotal number of pulses for the test period is divided by 100.

The test method of this invention is repeated by moving the electrolytecell to several locations on the surface of the anodized aluminum part.The figures of merit for all tests are averaged it relatively consistentand compared with a predetermined standard expressed in the same figureof merit units. Such standard is obtained by exposing tested articles touse exposure tests and correlating test values with the results of theexposure tests.

The invention as hereinbefore discussed has contemplated a resistance 19of fixed amplitude. In a second embodiment resistance 19 is increasedeither manually or automatically by conventional electrical controlmeans so as to maintain the current registered at milliamrn'etef 20 atan essentially even level. With the coating 14 undergoingelectrochemical deterioration, there is a tendency for an increase incurrent in the circuit with the applied voltage remaining constant. Withan increase in the value of re sistance 19 sufiicient to balance theresistance loss by coating 14, the current flow is stabilized and thevoltage drop across coating 14 decreases even more than when a fixedresistance 19 is employed. The optional recording and the integration ofvoltage across the coating with time are carried out in the same manneras before.

Anodized coatings may be evaluated by maintaining a constant voltage andintegrating the current through the coating with time. This testeliminates the need for resistance 19 of FIGURE 2. This method althoughsuggested by the voltage integration test aforedescribed is notequivalent to such test. In the voltage integration method heretoforedescribed, the voltage across the coating de-= cfeases pfoportionallywith a decrease in the electrical resistance of the coating as thecoating deteriorates thereby providing a more restrained electrochemicalattack upon the coating and causing a more controlled deterioration ofthe coating. In the current integration method, the voltage across thecoating remains essentially constant with increasing current since thereis no other significant resistance in the circuit and the deterioratingcoating is subjected to relatively more severe conditions in the laterstages of the test, or automatic means must be incorporated in thecircuitry to reduce the applied voltage at predetermined current levelsand allowance made for such changes in the recording and integrationunits to automatically compensate for the resultant distortion of thecurrent integral with time. If operated without change in the appliedvoltage, this test may be carried out with the circuitry illustrated inFIGURE 2 modified by replacing integration unit 26 with a unit providinga figure of merit equal or proportional to the ampere-seconds associatedwith the current through the coating during the test period. Theoptional voltage recorder 25 may be replaced with a current recorder.

I claim:

1. The process for evaluating a metal oxide coating upon a metal basewhich comprises placing said coating within an electrical circuitincluding integration means wherein said metal base serves as a cathodeof an electrolytic cell, said circuit comprising said base, an anode, anaqueous electrolyte in contact with said coating and said anode,impressing a constant difference of electrical potential upon saidcircuit which provides a resultant difference of electrical potentialacross said coating and measuring the electrical resistance of saidcoating with respect to time by integrating with said integration meansa function of said electrical resistance selected from the groupconsisting of electric current through said coating and difference ofelectrical potential across said coating over a predetermined period oftime.

2. The process for evaluating a metal oxide coating upon a metal basewhich comprises placing said coating within an electrical circuitincluding integration means wherein said metal base serves as a cathodeof an electrolytic cell, said circuit comprising said base, an anode, anaqueous electrolyte in contact with said coating and said anode,impressing a constant difference of electrical potential upon saidcircuit which provides a resultant difference of electrical potentialacross said coating and evaluating the corrosion resistance of saidcoating by integrating with respect to time, with said integrationmeans, for a predetermined period of time a function of the electricalresistance of said coating that changes with time during saidpredetermined period of time selected from the group consisting ofelectric current through said coating and difference of electricalpotential across said coating.

3. The process of claim 2 wherein said function that changes withrespect to time is the difference of electrical potential across saidcoating.

4. The process of claim 2 wherein said function that changes withrespect to time is the electric current through said coating and betweensaid anode and said base.

5. The process for evaluating a metal oxide coating upon a metal basewhich comprises placing said coating within an electrical circuitwherein said metal base serves as a cathode of an electrolytic cell,said circuit comprising integration means, said base, an anode, anaqueous electrolyte in contact with said coating and said anode, saidbase being in electrical connection with a first conductor terminal,said anode being in electrical connection with a second conductorterminal, said terminals being in electrical connection With anelectrical energy generation source adapted to apply a constant voltageacross said terminals, applying a constant voltage across said terminalsresulting in application of at least a portion of said voltage acrosssaid coating and evaluating the corrosion resistance of said coating byintegrating with respect to time. with said 6 integration means, for apredetermined period of time a function of the electrical resistance ofsaid coating that changes with time during said predetermined period oftime selected from the group consisting of electric current through saidcoating and difference of electrical potential across said coating.

6. The process of claim 5 wherein said circuit includes a resistance inexcess of about ohms in series with the electrical resistance of saidcoating and the function of said electrical resistance that changes withtime is the difference of electrical potential across said coating.

7. The process of claim 6 wherein said resistance is of fixed amplitude.

8. The process of claim 6 wherein said resistance is a variableresistance adjustable to maintain the resultant electric current throughsaid circuit at an essentially constant level.

9. The process of claim 5 wherein said circuit includes a resistance inexcess of about 10,000 ohms in series with the electricalresistance-between said anode and said base.

10. The process of claim 5 wherein said period of time is at least oneminute.

11. The process for evaluating a metal oxide coating upon a metal basewhich comprises placing said coating Within an electrical circuitwherein said metal base serves as a cathode of an electrolytic cell,said circuit comprising integration means, said base, an anode, anaqueous electrolyte in contact with said coating and said anode, and asignificant electrical resistance in series with the electricalresistance of said coating exterior to said electrolyte and evaluatingthe corrosion resistance of said coating by applying a constantdifference of electrical potential to said circuit providing a firstdifference of electrical potential across said significant resistanceand a second difference of electrical potential across said coating, andintegrating the difference of electrical potential across said coatingwith time for a predetermined period of time with said integrationmeans.

12. The process for evaluating an oxide coating upon and formed from analuminum base which comprises placing said coating within an electricalcircuit wherein said aluminum base serves as a cathode of anelectrolytic cell, said circuit comprising integration means, saidaluminum base, an anode, an aqueous electrolyte in contact with saidcoating and said anode, and a resistor providing a resistance in excessof about 100 ohms exterior to said electrolyte and in series with theelectrical resistance of said coating, evaluating the corrosionresistance of said coating by applying a constant difference ofelectrical potential across said circuit thereby providing a firstdifference of electrical potential across said resistor and a seconddifference of electrical potential across said coating which decreaseswith a decrease in the electrical resistance of said coating withresultant increase in electric current through said circuit, andintegrating the difference of electrical potential across said coatingwith time for a predetermined period of time with said integrationmeans.

13. The process of claim 12 wherein said predetermined time is at leastone minute.

14. The process of claim 12 wherein said resistor provides a resistancein excess of about 10,000 ohms.

15. The process of claim 12 wherein said coating is a coating producedelectrochemically in the presence of sulfuric acid.

16. The process of claim 12 wherein said integration is effected byconverting said second difference of electrical potential to differenceof electrical potential pulses, the frequency of which is a function ofthe difference of electrical potential across said coating with time,and impressing said pulses upon a counter.

17. The process for evaluating a metal oxide coating upon a metal basewhich comprises placing said coating Within an electrical circuitincluding integration means wherein said metal base serves as a cathodeof an electrolytic cell, said circuit comprising said base, an anode,and

7 in aqueous electrolyte in contact with said coating and ;aid anode,and evaluating the corrosion resistance of said :oating by impressing aconstant difference of electrical aotential upon said circuit whichprovides a resultant dif- Eerence of electrical potential across saidcoating and integrating the electric current through said coating withtime for a predetermined period of time with said integration means.

18. The process for evaluating an oxide coating upon and formed from analuminum base which comprises placing said coating Within an electricalcircuit including integration means wherein said metal base serves as acathode of an electrolytic cell, said circuit comprising said base, ananode, and an aqueous electrolyte in contact with said coating and saidanode, and evaluating the corrosion resistance of said coating byimpressing a constant difference of electrical potential upon saidcircuit which provides a resultant diiference of electrical potentialacross said coating and integrating the electric current through saidcoating with time for a predetermined period of time with saidintegration means.

19. The process of claim 18 wherein said period of time is in excess ofone minute.

References Cited by the Examiner UNITED STATES PATENTS 2,786,021 3/1957Marsh 204l 2,894,882 7/1959 Strodtz' 204-195 2,960,455 11/1960Frankenthal 204195 OTHER REFERENCES Brennert, Joun, Iron & SteelInstitute, volume (1937), pages l0lp-1l1p.

Campbell et al., Trans. of the Electrochemical Soc., volume 76 (1939),pages 303328.

May, Jour. of the Institute of Metals, No. 2 (1928), volume XL, pages147-175.

Miley, Carnegie Schlorship Memoirs, Iron and Steel Institute, volume 25,pages 201-208 (1936).

Price et al., Trans. of the Electrochemical 800., volume 76 (1939),pages 329-340.

Spooner, Technical Proc. of the 44th Annual Convention of the Am.Electroplaters Soc., June 20, 1957, pages 132-142.

JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.

